Due to the NH skin effect, the physics properties for 1D nonreciprocal SSH
model under open boundary condition (OBC) are characterized by \(\hat{H}_{\mathrm {OBC}}(k)\)  rather than \(\hat{H}_{\mathrm {PBC}}(k)\)[1]}.
Consequetly, the effective hopping parameters become \(\bar{t_{1}}=\sqrt{(t_{1}+\gamma )(t_{1}-\gamma )}\) , and \(\bar{t_{2}}=t_{2}\) . To characterize the
topological properties of the NH topological system, the non-Bloch topological
invariant \(\bar{w}\)  of \(\hat{H}_{\mathrm {OBC}}(k).\)  In the region of
\(\left|\bar{t}_{1}\right|<\left|\bar{t}_{2}\right|\) ,
\(\bar{w}=1,\)  the system is a topological insulator; In the region of
\(\left|\bar{t}_{1}\right|>\left|\bar{t}_{2}\right|\) ,
\(\bar{w}=0,\)  the system is a normal insulator. Quantum phase transition occurs
at \(\left|\bar{t}_{1}\right|=\left|\bar{t}_{2}\right|,\)  at
which bulk energy gap under OBC is closed.
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